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I'm hoping someone on this forum can help me with a "wood filler" question. My husband and I own an older home, built in the 50's, and it is sided with wide cedar clapboard siding and then stained with a cabots product solid stain, Mission Brown to be exact which is a dark brown. We have yet to find a wood filler product to fill nail holes once they have been set, and small cracks etc. that does not fall out due to shrinkage, or become shiny once painted, or not take paint at all. We live in CT where there are seasonal fluctuations which I know make it a bit trickier. I don't know if the fact that the house has a balloon construction makes any difference. I would truely be grateful for an expert opinion. Thanks. "Proud Mom on an Army Infantryman"

I'm quite surprised you're seeing enough movement in nail holes to loosen ready-made wood putties. But, here's a home-made filler that I use all the time that works well on exteriors...

Get some 2-part epoxy, not the 5-minute kind, anything that has an open time of an hour or more is what you want. Then get some saw dust, actually dust from a sander is better, but either will do. If you don't have your own, all the home centers have panel saws in the lumber section and you can get it there, just bring a couple of bagies and load up!

Then what you'll want to do is mix the epoxy per the directions and than add the saw dust. You actually want this to be more wood than epoxy - the epoxy is there just to hold things together and as long as the mixture is moist it will be fine. Then just use it as you would any wood filler. Give it 2x the cure time on the epoxy package before sanding or applying finish.

Because the epoxy contains a good percentage of real wood fibers, it takes stain and paint well... it also sands beautifully too.

I think you have correctly identified the nature of your problem: your siding is swelling and moving with the seasonal changes in heat and humidity. Your choice of a dark siding color hightens the problem as the surface temperature of the siding is considerably hotter than by a lighter color. A house of the 50's would have had minimal wall insulation and very little in the way of an interior vapor barrier to stop water vapor from entering the walls and contributing to the seasonal variations. Your choice of Cabot stain over a heavy body paint was a good idea. Stain breathes much better than paint, allowing moisture to migrate outward without popping a painted surface. I have used Cabot's stains for years. I do preferr the acrylic version over the oil for its color fastness. In situations where the siding was very dry and worn, I would use the oil version as a primer for the final coat of latex stain after about a week of drying time.

As to what to use: JLMCDANIEL hints as to what to use. You need a product with elasticity. Try googling "elastomeric wood filler products". There are many products out there by well known companies such as DAP designed to expand and contract with the seasonal movement of the wood. The key word is "elastomeric".